SEAFLOOR SPREADING

theory of lithospheric evolution that holds that the ocean floors are spreading outward from vast underwater ridges. First proposed in the early 1960s by the American geologist Harry H. Hess, its major tenets gave great support to the theory of continental drift and provided a conceptual base for the development of plate tectonics.

Discovery of the Mid-Ocean Ridges

Development of highly sophisticated seismic recorders and precision depth recorders in the 1950s led to the discovery in the early 1960s that the Mid-Atlantic Ridge, a vast, sinuous undersea mountain chain bisecting the Atlantic Ocean, was in fact only a small segment of a globe-girdling undersea mountain system some 40,000 mi (64,000 km) in length. In many locations, this mid-ocean ridge was found to contain a gigantic cleft, or rift, 20 to 30 mi (32–48 km) wide and c.1 mi (1.6 km) deep, extending along the crest of the ridge. The ridge itself does not form a smooth path, but is instead offset in many places. The offsets are called fracture zones, or transform faults. The ridge crest and its associated transform faults are the locus of nearly all shallow earthquakes occurring in mid-ocean areas. Continued study of the mid-ocean ridges is a major component of U.S. research in the global oceans.

Role of the Spreading Center

In 1962 Hess proposed that the seafloor was created at mid-ocean ridges, spreading in both directions from the ridge system. At the spreading center, liquid rock called basaltic magma rises from the earth's mantle as it upwells beneath the spreading axis. When the magma hardens, it forms new oceanic crust that becomes welded to the original crust. Spreading is believed to be caused by far-field stresses, and the upwelling of the mantle beneath the spreading axis is the passive response to plate separation. The oceanic trenches bordering the continents mark regions where the oldest oceanic crust is reabsorbed into the mantle through steeply inclined, earthquake-prone subduction zones. The pull of the deeply plunging lithosphere is one of the forces that may drive plate separation.

Supporting Evidence for Seafloor Spreading

Abundant evidence supports the major contentions of the seafloor-spreading theory. First, samples of the deep ocean floor show that basaltic oceanic crust and overlying sediment become progressively younger as the mid-ocean ridge is approached, and the sediment cover is thinner near the ridge. Second, the rock making up the ocean floor is considerably younger than the continents, with no samples found over 200 million years old, as contrasted with maximum ages of over 3 billion years for the continental rocks. This confirms that older ocean crust has been reabsorbed in ocean trench systems.

By the mid-1960s studies of the earth's magnetic field showed a history of periodic reversals in polarity (see paleomagnetism). A timescale for "normal" and "reversed" polarity was established, showing 171 magnetic "flip-flops" in the past 76 million years. Magnetic surveys conducted near the mid-ocean ridge showed elongated patterns of normal and reversed polarity of the ocean floor in bands paralleling the rift and symmetrically distributed as mirror images on either side of it. The magnetic history of the earth is thus recorded in the spreading ocean floors as in a very slow magnetic tape recording, forming a continuous record of the movement of the ocean floors. Other supportive evidence has emerged from study of the fracture zones that offset the sections of the ridge.

Bibliography

See J. Coulomb, Sea Floor Spreading and Continental Drift (1972).

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